This invention relates to an image forming apparatus with a unidirectional magnification function and, more particularly, to an image forming apparatus suitable for an electronic copying machine, comprising a variable magnification function which allows desirable reduction or enlargement of an image in size, and a unidirectional magnification function, in which an original image can be freely reduced or enlarged in size only in one direction.
In recent years, various electronic copying machines have been developed which comprise a variable magnification function that enables desired reduction or enlargement of an image in size. Operating principles of such a copying machine with a stationary original table will be described briefly.
An original placed on an original table is scanned by a scanning section comprising an exposure lamp and a mirror. The reflected light from the original is radiated through a plurality of mirrors and lenses onto a rotating photosensitive drum which is uniformly precharged. Thus, an electrostatic latent image is formed on the drum. The latent image is developed with toner, and the toner image is transferred to a paper sheet, thus completing one copying cycle.
In this case, the size of a copied image in a scanning direction of the original is determined by a ratio of scanning speed to a rotating speed of the photosensitive drum (to be referred to as a speed rate hereinafter). The size of the copied image in a direction perpendicular to the scanning direction is determined by a ratio of an optical path length from the original to a predetermined lens for guiding the reflected light on to the photosensitive drum, to an optical path length from the predetermined lens to the photosensitive drum (to be referred to as an optical path length ratio). Therefore, when the speed ratio and the optical path length ratio are changed, reduction or enlargement magnification for copying can be freely set with respect to an original image.
In the conventional copying machine, however, magnification in the original scanning direction (to be referred to as longitudinal magnification) is equal to magnification in a direction perpendicular to the scanning direction (to be referred to as lateral magnification), and one of these magnifications cannot be changed independently. For this reason, it is impossible to obtain a copied image which is reduced or enlarged in only the longitudinal or lateral direction of the original during an actual copying operation.
Another conventional copying machine has been proposed wherein, when optical system lenses are replaced with accessory unidirectional magnification lenses, a copied image which is reduced in size at different magnifications in the longitidinal and lateral directions can be formed. With this copying machine, however, lenses must be changed every time such an image is to be formed, resulting in inconvenience to a use. In addition, the copying operation is only enabled for fixed magnifications. Furthermore, a copied image which is reduced or enlarged in size in only one direction of the original cannot be obtained. Since a user must repeatedly change the lenses, they can easily be damaged.